Geochronology of the Cordillera Real granitoids, the inner magmatic arc of Bolivia

Abstract

The Real Cordillera granitoids are a suite of Triassic and Oligocene plutons located at the core of the Eastern Cordillera of the Central Andes of Bolivia. Its geotectonical setting, chemical and ore composition make them part of the so called “Inner Magmatic Arc” which differs from the actual “Magmatic Arc” located immediately to the west. U-Pb SHRIMP ages were obtained in order to constrain their crystallization ages. The Triassic group yielded the following results: 240 ± 2 Ma for the Huato granite, 230.7 ± 1.3 Ma for the Illampu granodiorite, 222.2 ± 2.4 Ma for the Huayna Potosí granite and 221.9 ± 1.5 Ma for the Taquesi granodiorite. For the Oligocene group we obtained two ages of 26.87 ± 0.26 and 26.88 ± 0.21 Ma both for the Quimsa Cruz granite. Mafic enclaves from the Illampu and Taquesi granodiorites report ages that were older than their respective granitoid hosts, yielding 234.1 ± 1.3 Ma and 227 ± 1.3 Ma, respectively. Secondary processes related to regional thermal anomalies and magmatic melt-enrichment, reset the K/Ar and U/Pb isotopic systems, producing: a) younger ages by Ar loss and b) older ages by U/Pb isotopic ratios reorganization. As noted in previous studies, the Zongo/Kuticucho Triassic granite yielded extremely high U enrichment in most zircon analysed, producing reset of U/Pb ratios, wide span in age ranges and reverse discordia curves that obscure its actual crystallization age. Relatively abundant zircon inheritance was found in these “cold” and inheritance-rich granitoids, with ages suggesting provenance from early Paleozoic metapelites that also recycled older sources. This relatively abundant xenocrystic inheritance records the influence of the Gondwanide orogeny (336-205 Ma) as an overall subduction arc environment, punctuated at its final stage with the imprint of a continental rifting (245-220 Ma).

Provenance study of Phanerozoic rocks from the Cordillera Real of Bolivia

“…In Figures 7 and 8, we compile U/Pb detrital zircon age from the database of Reimann et al (2010) along with our two samples, RIB-51 and RIB-57, and the inherited xenocrystal zircon ages of CRGs (Iriarte et al 2021). All the sedimentary formations are from the upper Ordovician.…”

“…220 Ma, with a younger pulse at ca. 27 Ma (Gillis et al 2006, Cordani et al 2019, Iriarte et al 2021. Their country rocks have early Paleozoic ages (Fig.…”

“…Source: modified from published maps of the Geological Service of Bolivia (GEOBOL 1994(GEOBOL , 1995(GEOBOL , 1997(GEOBOL , 2013. U/Pb SHRIMP dating (Gillis et al 2006, Iriarte et al 2021) revealed a protracted history of magmatism of nearly 60 Ma, starting at ca. 280 and ending at ca.…”

“…Table 1 shows that a total of 64 zircon xenocrysts was recovered from the plutons studied by Iriarte et al (2021). Twelve of them are younger than the Ordovician Amutara Formation and, therefore, must be considered as grains ultimately inherited from igneous intrusions related to the Famatinian or Gondwanan magmatic rocks.…”

“…SHRIMP U, Th/U, percentage of Concordia discordance, dates ± 1 sigma error (Ma), isotopic ratios ± 1% error from the granitoids of Cordillera Real(Iriarte et al 2021). Ranges from 298 to 479 Ma, n = 20 (in italic) were excluded for statistical comparison with the Ordovician units.…”